This application claims the priority of German Application No.: 101 43 806.0-13 filed Sep. 6, 2001,the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an exhaust-gas cleaning system for an internal combustion engine, in particular for a motor vehicle, having a catalytic converter housing with an associated exhaust catalytic converter and an inlet opening for connecting an exhasut pipe.
The published specification German Patent document DE 31 20 212 A1 has disclosed a device through which gas flows and which has an inlet funnel with indentations lying opposite one another. The indentations are used to make the flow more uniform or to achieve a desired flow distribution over the gas inlet cross section without significant losses occurring. Filters or catalytic treatment devices, otherwise known as exhaust catalytic converters, are primarily suitable as the device through which gas flows. In addition, a very wide range of diffusers are known and are likewise used to make gas flows more uniform while avoiding a significant pressure drop.
Exhaust catalytic converters in motor vehicles are on the one hand exposed to particular thermal and mechanical loads, especially when they are fitted close to the engine, but on the other hand are to have a high activity even during unfavorable operating conditions on the part of the engine, for example at low exhaust-gas temperatures. Particularly in the event of a high load, the engine exhaust gases are at a high temperature and may cause undesirable catalyst ageing. Moreover, under these conditions the engine exhaust gases contain incompletely burnt constituents, on account of mixture enrichment, and a certain residual oxygen content, on account of incomplete combustion. Therefore, the result is a reactivity of the exhaust gas, which can lead to further exothermic reactions taking place in the catalytic converter, in particular in its entry region. The associated release of heat makes the thermal load on the catalytic converter more intense and can cause its destruction. If exhaust catalytic converters are installed close to the engine, in the event of high gas velocities they are furthermore subjected to mechanical loads from gas surges. These gas surges result from load changes in the engine and have an erosive and abrasive effect on the catalytic coating of the exhaust catalytic converter, which likewise reduces its service life. Moreover, for a good action under unfavorable operating conditions, for example in the event of an engine cold start or when the engine is warming up, a high level of catalytic converter activity is required, and consequently optimum catalyst utilization and/or uniform distribution of the exhaust-gas flow over the catalytic converter inlet surface needs to be ensured.
It is an object of the invention to provide an exhaust-gas cleaning system which allows improved and more reliable operation of the associated catalytic converter.
According to the invention, this object is achieved by an exhaust-cleaning system for an internal combustion engine, in particular for a motor vehicle, having a catalytic converter housing with associated exhaust catalytic converter and an inlet opening for connecting an exhaust pipe, wherein a throttle element is arranged in the catalytic converter housing, in such a manner that a first reaction chamber, which lies between the throttle element and the inlet opening, and a second reaction chamber, which lies between the throttle element and the exhaust catalytic converter, are formed by the throttle element.
According to certain preferred embodiments of the invention, a throttle element is arranged in the catalytic converter housing of the exhaust-gas cleaning system, in such a manner that a first reaction chamber, which lies between a throttle element and inlet opening of the catalytic converter housing, and a second reaction chamber, which lies between the throttle element and the exhaust catalytic converter, are formed by the throttle element. Further reactions of incompletely burnt fuel constituents with residual oxygen contents in the exhaust gas may take place in these reaction chambers. Unlike with a diffuser, the throttle element on the one hand causes a certain build-up of the gas stream upstream of the throttle element in the first reaction chamber, and on the other hand mixing and swirling of the gas stream is effected downstream in the second reaction chamber.
In one configuration of preferred embodiments of the invention, the throttle element is designed as a diaphragm with a uniform perforation. This design of the throttle element results in uniform mixing of the exhaust gas in the second reaction chamber, which improves the way in which the subsequent reactions take place, and effects a uniform distribution of the gas flow over the catalytic converter entry surface and produces uniform absorption of gas surges.
In a further configuration of preferred embodiments of the invention, the throttle element has a wall which is curved oppositely to the direction of flow. This firstly results in increased mechanical stability and secondly allows the thermal expansions, which are inevitable as a result of the temperature load, to be dealt with in an advantageous way.
In a further configuration of preferred embodiments of the invention, the throttle element is designed as a perforated metal sheet comprising high-temperature resistant and/or catalytically active metal. Designing the throttle element as a perforated metal sheet is particularly advantageous for manufacturing technology reasons. On account of the high thermal load on the catalytic converter housing which is fitted mainly in the region close to the engine, the perforated metal sheet is produced, for example, from a high-alloy steel. High-alloy steels often have catalytic properties, so that subsequent reactions at the surface of the perforated metal sheet are promoted. A suitable choice of the metal material allows advantageous utilization of its catalytic properties.
In a further configuration of preferred embodiments of the invention, the throttle element is designed as a thermal shield between the inlet opening of the catalytic converter housing and the catalytic converter. On account of the separation between the first and second reaction chambers, the throttle element acts as a shield against the release of heat caused by subsequent reactions in the first reaction chamber. The heat which is released is dissipated via the throttle element to the wall of the catalytic converter housing, and moreover the direct action of thermal radiation which originates from the first reaction chamber on the catalytic converter is avoided.
Further features and combinations of features will emerge from the description and the drawing. Specific exemplary embodiments of the invention are illustrated in simplified form in the drawing and explained in more detail in the description which follows.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.